1. Field of the Invention
The invention pertains to improved electrooptical devices for switching pluralities of unpolarized or polarized optical signals between input and output multimode single-strand fiber optical guides and more particularly relates to novel electrooptical switch elements and matrices employing electric field-effect liquid crystal compositions adaptable for the construction of large scale, integrated multiple switch arrays.
2. Description of the Prior Art
Best known in the prior art are optical switching devices employing electrooptical materials such as LiNbO.sub.3 or LiTaO.sub.3. While some success has been achieved in devising electrical field switched devices in matrix forms, the use of such solid crystalline materials imposes serious limitations. Generally, the size of available LiNbO.sub.3 or LiTaO.sub.3 crystals is seriously limited, and there is no apparent opportunity seen for overcoming this size limitation without extreme expense. No other known solid electrooptical materials have large electrooptic coefficients and otherwise suitable properties. With array substrate sizes limited in this manner, the number of switch elements that may be employed in matrix switches constructed of such materials is severely limited.
On the other hand, it has also been proposed to use nematic liquid crystal materials in unit optical switches; but, generally, the single switch configurations proposed have inherent geometric and other properties making them not particularly useful for combination in simple optical multiple switch matrices. For example, a liquid crystal switching matrix that might be based on the unit switch described in the technical paper "Nematic Crystal Digital Light Deflector" by G. Labrunie and S. Valette, beginning on page 1802 of volume 13, number 8 in the August 1974 issue of Applied Optics, would demonstrate serious defects. Though the Labrunie et al paper might suggest a honeycomb matrix configuration of optical switches made up of identical modular elements, it would be nonetheless very expensive and of doubtful competitiveness because all contiguous faces of the unit switches must be accurately ground with high optical precision, not only for achieving high optical flatness, but also so as to assure that the angles between active faces are all precisely equal and that there are no air spaces. Any departure from such precision would result in high optical losses, greatly increased scattered light, and consequent general degradation of efficiency of the matrix. This major defect of the imagined honeycomb type or organization yields only one active switching region for each elemental switch module and it is, therefore, apparent that such a strictly modular approach is not economical from at least a second viewpoint. In fact, the idea that the strict modularity of the unit hexagonal cells be retained is seen to be the seat of the aforementioned problems. Nonetheless, liquid crystal materials are characterized by large changes in optical index of refraction for only modest changes in the electric field at relatively low electric field magnitudes and have other significant advantages, as are discussed widely in the literature and, for instace, in the U.S. patent application Ser. No. 947,983, now U.S. Pat. No. 4,201,442, by D. H. McMahon and R. A. Soref for a "Liquid Crystal Switching Coupler Matrix", filed Oct. 2, 1978 and assigned to Sperry Corporation.
The faults of the prior art are further discussed in the R. A. Soref U.S. patent application Ser. No. 13,095 for "Liquid Crystal Switching Matrices", filed Feb. 21, 1979 and assigned to Sperry Corporation. The Soref invention comprises novel electrooptical matrix switches for selectively switching pluralities of optical signals between collimating output fiber light guides of the multimode, single strand kind. Switching is accomplished by selective electric field excitation of positive or of negative anisotropy liquid crystal media. The geometry of the multiple switch structure may be regarded as a hybrid between the modular and non-modular; in certain forms of the invention, sets of similar trapezoidal prismatic glass parts are economically employed and, between those glass prisms, thin planar liquid crystal layer cells are defined which may include a plurality of active electrooptic regions. The complete configuration may have one or more degrees of geometric symmetry. The assembly of prisms and liquid crystal layers may cooperate, for example, in directly light signals from three inputs to selected ones of three outputs, while certain of the prisms cooperate with light absorbers for removing unwanted scattered light.